Sexual transmission of HIV is increasing in the South among women, minorities, and men who have sex with men. Moreover, in several recent studies, HIV incidence among injecting drug users was associated more with sexual risk behaviors than injecting risk behaviors. The increasing incidence of HIV associated with sexual transmission may reflect the success of interventions in reducing injecting risk, and the fact that sexual risk reduction is often influenced by social, gender, and cultural norms in addition to individual motivation. Furthermore, patterns of drug use have shifted in the South, with high rates of crack use among minorities and within rural areas. The proposed 5-year multimethod longitudinal study will identify factors associated with the sexual diffusion of HIV and other sexually transmitted infections (STIs) within drug-using groups and from drug users to non-drug users in rural and urban settings in North Carolina. The approach will combine ethnographic observations, individual behavioral interviews, social network analysis, biological testing, and structural environmental assessments. These data will provide inputs for developing conceptual and mathematical models of HIV epidemics among drug users in rural and urban settings. Such models can aid in intervention development by helping to determine which modifiable behavioral and structural factors would have the greatest impact on reducing HIV and STI transmission. The proposed team will work collaboratively with the State HIV/AIDS office and the University of North Carolina (UNC) Center on AIDS Research Virology Core for virological testing to identify recent HIV infections. The specific aims of the study are as follows: 1. To assess the effects of environmental (e.g., neighborhood characteristics, rural-urban setting, and syringe access), social network (e.g., density, size, and stability), and personal (e.g., age, race, and gender) characteristics on individual risk behavior, prevalence, and incidence of HIV and STIs. 2. To identify and describe differences in social network characteristics associated with urban-rural settings, social structure, risk behaviors, geographic location, and environmental conditions and to link these differences to the spread of HIV and STIs within and between networks. 3. To develop a conceptual model of HIV and STI transmission within, between, and outside the networks. This model will be based on the findings in Aims 1 and 2 and include biological, personal, network, and environmental components and their interaction with risk behavior. 4. To develop a mathematical model that will operationalize the conceptual model in Aim 3. This model will synthesize findings from Aims 1 through 3 in order to estimate (through simulation) the impact of intervening at different levels (biological, individual, network, and environment) and at specific points within levels.